The position of a mobile station, such as, for example, a cellular telephone, may be estimated based on information gathered from various systems. One such system may comprise the Global Positioning System (GPS), which is one example of a satellite positioning system (SPS). SPS systems such as GPS may comprise a number of space vehicles (SV) orbiting the earth. Another example of a system that may provide a basis for estimating the position of a mobile station is a cellular communication system comprising a number of aerial and/or terrestrial base stations to support communications for a number of mobile stations.
A position estimate, which may also be referred to as a position “fix”, for a mobile station may be obtained based at least in part on distances or ranges from the mobile station to one or more transmitters, and also based at least in part on the locations of the one or more transmitters. Such transmitters may comprise SVs in the case of an SPS and/or terrestrial base stations in the case of a cellular communications system, for example. Ranges to the transmitters may be estimated based on signals transmitted by the transmitters and received at the mobile station. The location of the transmitters may be ascertained, in at least some cases, based on the identities of the transmitters, and the identities of the transmitters may be ascertained from signals received from the transmitters.
In a Code Division Multiple Access (CDMA) digital cellular network, the position location capability can be provided by Advanced Forward Link Trilateration (AFLT). In Wideband Code Division Multiple Access (WCDMA) and Long Term Evolution (LTE) networks the position location capability is provided by Observed Time Difference Of Arrival (OTDOA). These techniques compute the location of the mobile station (MS) from the mobile station's measured time of arrival of radio signals from the base stations. A more advanced technique is hybrid position location, where the mobile station employs a Global Positioning System (GPS) receiver and the position is computed based on both AFLT (or OTDOA) and GPS measurements.
LTE OTDOA positioning technology uses Positioning Reference Signals (PRS) to measure Time Of Arrival (TOA) from each neighboring cell in order to calculate OTDOA position fix. The accuracy of OTDOA position fix depends on accuracy of individual PRS measurements and the number of measured neighbor cells. In order to be able to measure the PRS signals from the serving cell and neighbor cells, user equipment (UE), or mobile device, sends an Assistant Data Request to an OTDOA system server. The Server will then send a suite of cells' information (e.g., BSA and Timing info) to the UE. Usually, for UE on a single frequency, the server can send the information of one serving cell plus up to 24 neighbor cells. Some conventional location server systems include fairly rudimentary logic for selecting neighbor cells to be included in the OTDOA Assistance data. For example, some systems use the pre-fix position of the UE, find all the neighbor cells within certain distance (say 50 km), and then select whatever cells appear first in the list.
However, due to the PRS pattern in LTE and the carrier's site plan, there is a good chance that among the selected neighbor cells, some of them will have strong interference to or from the serving cell which makes them poor choices for OTDOA positioning purposes.